1. Field of the Invention
This invention relates to controlling the gain of RF power amplifiers. Specifically, an FET power amplifier is caused to have substantially constant operating characteristics by maintaining the amplifier current constant for a given output power in spite of fluctuations in ambient temperature, load, and supply voltage.
2. Description of the Prior Art
FIG. 1 illustrates a prior art transmission circuit of a radio transmitter (e.g., a battery powered transmitter such as an automobile telephone). Amplifier stages 1, 2, 3 are each conventional FET (field effect transistor) amplifiers including first stage pre-amplifier 1, second stage driver amplifier 2 and final stage amplifier 3. A common power supply voltage V.sub.DD is provided from a terminal 4 to the drain terminals of amplifier stages 1, 2, and 3. A fixed gate bias voltage V.sub.G is provided from terminal 5 to the gate terminals of amplifier stages 1, 2. An example of such an amplifier is the Fujitsu FMC080802-21 power amplifier.
An RF (radio frequency) signal Sig In is provided at input terminal 6 to the first amplifier stage 1. Sig In is amplified to a predetermined level sequentially by amplifier stages 1, 2, 3; the output of stage 3 is transmitted by antenna 7 to a receiver (not shown). A bias control voltage provided at terminal 8 to the gate terminal of final stage amplifier 3 has a magnitude controlled by additional circuitry (not shown) typically to be constant, or is varied to be a function of the level of Sig In. Specifically, when the level of Sig In is small, the bias control voltage is varied to shift the operating point of amplifier stage 3 to reduce the drain current of stage 3. This bias control decreases power consumption of the amplifier and is well known in the field of battery powered portable telephones.
Generally, the gain of the amplifier of FIG. 1 is a function of ambient temperature. When the temperature rises, the drain current of the FET in each stage is reduced, causing a subsequent decrease in gain. This is shown in FIG. 2 which is an operating characteristic diagram showing the relation of gain and drain current versus bias control voltage in the power amplifier of FIG. 1 at two temperatures. Curve G.sub.1 of FIG. 2 shows gain in dB (vertical axis) versus bias control voltage (horizontal axis) at room temperature. Curve I.sub.D1 indicates variation in the drain current in mA at room temperature as a function of control voltage. Points A and C indicate gain and current at maximum transmission power (bias control voltage a), and points B and D represent gain and current at minimum transmission power (bias control voltage b). At point B, where transmission power is at a minimum, the drain current drawn (and hence power consumed) by the amplifier are also at a minimum. As transmission power is increased, the drain current must also be increased to prevent saturation and therefore loss of linearity and gain of the amplifier.
Curves G.sub.2 and I.sub.D2 are the operating characteristic diagram of the amplifier of FIG. 1 at an increased ambient temperature, corresponding to respectively curves G.sub.1 and I.sub.D1 at room temperature. Point B' on curve G2 represents the gain of the amplifier at minimum transmission power, and point D' on curve I.sub.D2 represents the drain current at minimum transmission power. Point A' on curve G2 represents the gain of maximum transmission power, and point C' on curve I.sub.D2 represents current at maximum transmission power.
The reduction in both gain and current is readily seen by comparing points A, B, C, D at room temperature to points A', B', C', D' respectively at the increased temperatures.
FIGS. 3a-3d illustrate the mechanisms which cause the fluctuations seen in FIG. 2. FIGS. 3a and 3b are characteristic diagrams of the drain voltage (horizontal axis) versus the drain current (vertical axis) as a function of gate voltage of the FET in amplifier stage 3. In amplifier stage 3, a load curve operating point is determined by the gate voltage and drain voltage so as not to cause saturation at maximum transmission power. Different drain and gate voltages cause different drain currents, and hence different operating points.
FIG. 3a represents the maximum transmission operating point at room temperature. The gate bias voltage V.sub.G chosen must correspond to a relatively large drain current so that the desired maximum transmission power can be achieved. FIG. 3b represents the minimum transmission operating point at room temperature where the large current drawn is no longer necessary to achieve the desired output power. In order to reduce the D.C. power drawn by the amplifier, the gate bias voltage (V.sub.G) has been decreased. The maximum peak RF output voltage is represented by the intersection of the load curve of the drain voltage axis. The reduction in maximum transmission power between the two operating points can be seen by noting that in FIG. 3a the intersection point is V.sub.D4, while in FIG. 3b the intersection point is V.sub.D1 which is lower than V.sub.D4. This reduction also corresponds to the loss of gain as the gate bias voltage is lowered.
FIG. 3c represents the maximum transmission operating point at an increased temperature. Notice, compared to FIG. 3a, that given the same gate bias voltage (V.sub.G) the drain current has decreased and the maximum peak RF output voltage (as indicated by the intersection of the load curve to the drain voltage axis) has also decreased. The reason for the changes is that due to the increase in temperature, the load curve of FIG. 3c has moved parallel to the original load curve toward the intersection of the drain voltage and drain current axis. This change with respect to temperature is a general characteristic of all FET's.
FIG. 3d represents the minimum transmission operating point at an increased temperature. Similarly the maximum transmission point and reduction in current and maximum peak RF transmission voltage can be seen by comparing FIG. 3d to FIG. 3b.
The above diagrams and description deal with an increase in ambient temperature; the inverse situation occurs for an ambient temperature decrease.
As described above, fluctuations in ambient temperature cause variations in the drain current, which in turn cause variations in gain in prior art amplifiers. There is a strong correlation between variations in gain and variations in amplifier FET drain current as well as saturation point. Such variations in gain are undesirable in a cellular telephone system. In a digital cellular telephone system such as a CDMA system, it is required that a signal received at the base station be controlled with great accuracy to achieve maximum capacity of the system. It is therefore undesirable that transmitter gain vary with temperature.
Gain also varies due to fluctuations both in the power supply voltage and in the load. In such cases, there is also a strong correlation between variation in gain and variation in the drain current, so these variations can be compensated for in the same manner as the temperature variation.